Weldability of hardenable silicon alloyed spring steel.

One serious concern, related to high strength steels, is their weldability. An interesting high strength steel group contains a carbide free bainitic microstructure. This special microstructure, consisting of ferrite plates and stabilized austenite, is achieved by austempering a steel containing suitable amounts of Si and / or Al. These elements will suppress the formation of carbides resulting in an ausferritic structure. In this work, different pre- and post-weld treatments have been tested in order to avoid the formation of martensite in the weld and the heat affected zone of a medium carbon silicon- alloyed spring steel, 55Si7, in austempered condition. High power fibre-laser equipment has been used for the welding operation and furnace- and induction- heat treatments have been utilized to produce the pre- and post weld heating cycles. The resulting microstructures were investigated by optical- and scanning electron microscopy and the microhardness values and profiles for the different microstructures were measured. The most promising results were achieved by a combination of a pre- and post-weld heat treatment at 300oC and a heat treatment method based on the quenching and partitioning concept, resulting in hardness values as low as 500 and 430 HV, respectively. In comparison to this, welding without heat treatment resulted in a martensitic microstructure with hardness values of 800 HV.Godkänd; 2010; 20150326 (esa